1. Field of the Invention
The invention is directed to the field of microwave and optical waveguides. More particularly, the invention is directed to methods for designing compact mode control and converter elements for use with microwave and optical waveguides.
2. The Background Art
Mode control and conversion devices find wide application, for example, in microwave heating of plasmas for fusion experiments, multi-mode feeds for RADAR systems, microwave waveguide transitions, couplers and mode filters, microwave waveguide mode launchers for waveguide transmission systems and for laboratory demonstrations, microwave heating systems and optical waveguide couplers, mode converters and mode filters optical and microwave gratings, holographic elements, filters, phase shifters and the like.
Mode converters have been extensively used to convert outputs of high power microwave sources into lower order modes for plasma heating and low loss microwave transmission.
Currently, periodic gratings are used for conversion of modes in highly overmoded circular waveguides. These gratings are formed by periodically varying the waveguide radius resulting in a rippled wall structure and are usually analyzed by coupled mode theory. Such rippled wall structures result in very slight periodic field perturbations. Very high efficiencies have been reported for these gratings, but their lengths remain large compared to the waveguide transverse dimension. Various techniques have been implemented to reduce the length of these gratings, but the overall conversion length remains limited by the grating period. Minimal scattering occurs in such designs and a minimum achievable conversion length appears to be equal to one grating period. See, e.g., K. Kumric, et al., xe2x80x9cOptimization of Mode Converters for Generating the Fundamental TE01 Mode from TE06 Gyrotron Output at 140 GHz,xe2x80x9d International Journal of Electronics, Vol. 64 (January 1988), pp. 77-94, and M. J. Buckley et al., xe2x80x9cCompact Quasi-Periodic and Aperiodic TEON Mode Converters in Overmoded Circular Waveguides for use with Gyrotrons,xe2x80x9d I.E.E.E. Transactions of Microwave Theory and Techniques, Vol. 38, No. 6 (June, 1990), pp. 712-721. Similar structures in the form of gratings have been designed for optical waveguides. See, e.g., D. Marcuse, xe2x80x9cMode Conversion Caused by Surface Imperfections of a Dielectric Slab Waveguide,xe2x80x9d Bell Systems Technical Journal (December, 1969) pp. 3187-3215. All of these converter designs are relatively lengthy when compared to the radial dimension of the waveguide. A typical periodic grating mode converter is diagrammed in FIG. 1. In FIG. 1 a first input electromagnetic wavefront 10 is applied to the converter 12. The first wavefront 10 is formed of one or more modes. After interaction with converter 12, which is much longer than it is wide, a second electromagnetic wavefront 14 of the selected modality is output from converter 12. Such converters are typically 95-99.5% efficient.
Mode filters for high power microwave sources whose output power is distributed in various modes, permit extraction of a single mode at the output. Previous designs have not proven themselves particularly efficient. See, e.g., J. P. Tate, et al., xe2x80x9cExperimental Proof-of-Principal Results on a Mode-Selective Input Coupler for Gyrotron Applicationsxe2x80x9d, I.E.E.E. Transactions on Microwave Theory and Techniquesxe2x80x9d, Vol. 42, No. 10 (October 1994), pp. 1910-1917, and U.S. Pat. No. 3,771,078 dated Nov. 6, 1973 to H. G. Kidner, et al.
Mode launchers for efficiently exciting a specific mode into an overmoded waveguide can be difficult to construct. However, for a single mode waveguide only one mode survives and thus the mode purity is ensured.
Waveguide adapters, such as tapered sections, are generally used to join waveguides of unequal radial dimensions. Due to a gradual taper these devices are very long as compared to the radial waveguide dimension. See, e.g., W. A. Huting et al., xe2x80x9cNumerical Solution of the Continuous Waveguide Transition Problem,xe2x80x9d I.E.E.E. Transactions on Microwave Theory and Techniques, Vol. 36, No. 11 (November 1989), pp. 1802-1807.
Grating couplers can be used to couple free space light into an optical waveguide or vice versa and also for coupling between adjacent waveguides. See, e.g., Nishihara, Haruna and Suhara, xe2x80x9cOptical Integrated Circuitsxe2x80x9d, McGraw-Hill Optical and Electro-optical Engineering Series, 1989.
Accordingly, a method for designing more compact, yet equally efficient mode converters and control elements for microwave and optical applications would be highly desirable.
The present invention is directed toward the design of an aperiodic grating for mode conversion (defined as any operation on an input set of modes) which is based upon an inverse scattering optimization procedure. In accordance with this method, no predetermined shape of the grating structure is assumed. A constrained domain of all surfaces is searched to find an optimum aperiodic conversion surface profile for maximum conversion efficiency into the required output mode(s). Accordingly, the present method results in the design of rough surfaces or non-homogeneous structures for mode conversion. Because this method relies on scattering produced by short, forceful field perturbations, it is possible to achieve very small conversion lengths which are much less than one grating period. Precisely because periodic structures are not initially assumed, compact or rough structures are generally obtained. This method can be used to design mode converters, mode filters, waveguide adapters, mode launchers, power splitters and combiners, phase shifters and aperiodic gratings for use as couplers in optical waveguides. Efficiencies close to the theoretical maximums are obtainable with this method.
The structures achieved by this method have a narrow bandwidth of operation and therefore are suitable to the design of filters.
It is known that whenever an obstruction is placed in the path of an electromagnetic wave, the energy is scattered into various modes. Such an obstruction can be created in a waveguide by varying its dimensions or by changing the material of the dielectric inside it. The method of the present invention is directed at finding an optimum scattering surface that will scatter essentially all of the energy in the input mode(s) into one mode or a set of modes desired at the output. The following steps are used to design such a surface:
1. Specify the application of the device (i.e., the frequency of operation, type of device, structure, and size of the input and output waveguides, the mode composition of the incident electromagnetic field and the required mode composition of the output electromagnetic field);
2. Pick a method of variation (i.e., variation of waveguide shape only, variation of only the material properties of the obstruction or variation of both the shape and the material);
3. Choose the material to be used for the obstruction;
4. Decide the directions in which the obstruction and/or waveguide shape and material must vary;
5. Pick a suitable basis function to represent the waveguide shape and/or material properties in each of the waveguide dimensions;
6. Choose an initial structure approximation (i.e., give some arbitrary (but realistic) values to all the variables (coefficients) in the series representations and pick a length, L, for the structure);
7. Formulate the forward solution;
8. Perform a global optimization to find values of the coefficients that maximize the output power in the desired output mode(s);
9. (Optional) Obtain multiple designs (i.e., repeat steps 5 through 8 to come up with a number of designs and select an optimal design under the circumstances); and
10. Fabricate the device.
Accordingly, it is an object and advantage of the present invention to provide a method for designing compact, efficient waveguide mode control and conversion devices useable in microwave and optical applications.
It is a further object and advantage of the present invention to provide a method for designing mode converters, mode filters, waveguide adapters, mode launchers, power splitters, phase shifters and aperiodic gratings for use as couplers in waveguides.
It is a further object and advantage of the present invention to provide a methodology for creating aperiodic mode control and mode conversion surfaces which are more compact than conventional periodic structures used for these purposes.
Yet another object and advantage of the present invention is to provide a method for the design of narrowband filters, specifically for optical applications.
These and many other objects and advantages of the present invention will become apparent to those of ordinary skill in the art from a consideration of the drawings and ensuing description of the invention.